1. Field of the Invention
The present invention generally relates to touch panels, and more particularly, to a pen input type analog resistance film touch panel used as an input device that is disposed on a display surface of a liquid crystal panel of, for example, a PDA (Personal Digital Assistant) or a notebook computer, and on which input device an input is directly made on a display surface thereof by using a pen. The pen input type analog resistance film touch panel uses an analog resistance film method as the detection principle of X-Y coordinate, and uses a general detection method for detecting the coordinate of an input point.
2. Description of the Related Art
In electronic devices that are required to have small sizes, such as PDAs and notebook computers, it is also required to have a small touch panel. In order to reduce the size of a touch panel, it is desirable to use a general detection method for detecting the coordinate of an input point, since it is possible to reduce a dead space around a glass board. In the general detection method for detecting the coordinate of an input point, the potential gradient of a transparent conductive film on a film is used for detecting the X coordinate of the input point, and the potential gradient of a transparent conductive film on a glass board is used for detecting the Y coordinate of the input point.
FIGS. 1 and 2 show a conventional pen input type analog resistance film touch panel (hereinafter simply referred to as “the touch panel”) 10 that uses a general detection method for detecting the coordinate of an input point. In FIG. 1, arrows X1 and X2 represent the directions (X1-X2 directions) along the longer side of the touch panel 10, and arrows Y1 and Y2 represent the directions (Y1-Y2 directions) along the shorter side of the touch panel 10. In the touch panel 10, a film 20 is bonded to the top surface of a quadrangular glass board 11, which is a substrate, by a double-faced adhesive tape 19. The film 20 covers the glass board 11 via an air layer 18 (FIG. 2). A flexible cable 16 extends from the top surface of the glass substrate 11. A transparent conductive film 12 is formed on the top surface of the glass board 11. Dot spacers 13 (FIG. 2) are formed thereon in a dispersed manner. Elongated electrodes 14 and 15 are formed on the top surface of the glass board 11 along the sides of the glass board 11 opposing in the Y1-Y2 directions. In the film 20, a transparent conductive film 22 is formed on and adheres to the bottom surface of a PET (Polyethylene Terephthalate) film body 21. Additionally, an acrylate resin layer 23 is formed on the top surface of the PET film body 21 as a hard coat layer so as to protect the surface of the PET film body 21. Further, elongated electrodes 24 and 25 are formed along the sides of the film 20 opposing in the X1-X2 directions.
As shown in FIG. 2, the touch panel 10 is mounted on the top surface of a liquid crystal panel 30 and the periphery of the touch panel 10 is covered by a cover 31, thereby constructing a PDA 10A. By using a pen 35 made of polyacetal, an operator of the PDA 10A writes characters on the top surface of the film 20 and/or performs an input operation of pressing a predetermined portion of the top surface of the film 20.
When the pen 35 is pressed against the top surface of the film 20, the film 20 is bent as indicated by two-dot chain lines in FIG. 2, and the transparent conductive film 22 of the film 20 contacts the transparent conductive film 12 on the glass board 11 at the portion (contact point) where the pen 35 is pressed against.
When a voltage is applied between the elongated electrodes 14 and 15, a potential gradient in the X1-X2 directions is established by the transparent conductive film 12 on the glass board 11, the potential of the contact point is detected via the transparent conductive film 22 of the film 20, and the X coordinate of the contact point is determined from a predetermined mathematical expression representing a partial pressure.
When a voltage is applied between the elongated electrodes 24 and 25, a potential gradient is established in the Y1-Y2 directions by the transparent conductive film 22 of the film 20, the potential of the contact point is detected via the transparent conductive film 12 on the glass board 11, and the Y coordinate of the contact point is determined from a predetermined mathematical expression representing a partial pressure.
Since the transparent conductive films 12 and 22 are hard materials, peeling and/or cracking tend to occur due to a physical operation. The film 20 is bent in a portion against which the pen 35 is pressed. Hence, a tensile stress is applied to the transparent conductive film 22 of the film 20 at the bent portion. Thus, during long usage of the PDA 10A, peeling and/or cracking gradually occur. When peeling and/or cracking occurs in the transparent conductive film 22 of the film 20, resistance distribution of the transparent conductive film 22 varies, and an error is generated between the X coordinate of a position against which the pen 35 is pressed and the X coordinate that is output from the touch panel 10 as a signal. Consequently, the linearity, which is one of the characteristics required for the touch panel 10, is decreased, i.e., the numerical value representing the linearity is increased. The linearity required by markets is, for example, 1% or less (see FIG. 3, discussed in greater detail below). Thus, later in the service period of the PDA, a reduction of the linearity appears. When the linearity decreases to such an extent that it does not meet the specification, the PDA 10A starts to perform an operation different from that instructed by the pen 35, which is the end of the life cycle (service life) of the touch panel 10. As long as the PDA 10A is normally used, the life cycle of the touch panel 10 is sufficiently long, and there is no particular problem.
Through observation of people using PDAs, the inventor of the present invention has found that, while making a call, for example, some people reciprocally and repeatedly rub the same portion of the touch panel 10 by the pen 35 along an edge of the cover 31 as if they are drawing straight lines and not for performing an input operation. Such operations are severe operations that damage the touch panel 10.
When such severe operations are performed on the touch panel 10, peeling and/or cracking may occur in the transparent conductive film 22 of the film 20 in a relatively early stage. As a result, the linearity may be decreased and the touch panel 10 may come to the end of its life cycle earlier than expected.
Referring to FIG. 1 again, a touch panel protection sheet 40 (hereinafter referred to as “the protection sheet 40”) is one of the optional components for the PDA 10A. As shown in FIG. 2, the protection sheet 40 is used by being applied to a surface of the touch panel 10 so as to prevent the surface of the touch panel 10 from being scratched or contaminated. The protection sheet 40 is particularly used for, for example, terminals installed in the outside where the environment is severe, terminals for fast-food restaurants, or terminals for kitchen. When the protection sheet 40 is contaminated, the protection sheet 40 is peeled away and replaced with a new one. A user of the PDA 10A applies and peels away the protection sheet 40. The conventional protection sheet 40 includes an adhesive layer 41 on an entire surface thereof. Hence, there is a problem in that, upon application of the protection sheet 40, air bubbles containing air therein tend to be formed and the air bubbles become conspicuous. In addition, the conventional protection sheet 40 merely protects the surface of the touch panel 10 and does not provide other functions.